Sauté first 3 ingredients in hot oil in a large saucepan until tender. Add 1 cup chicken broth. Bring to a boil over medium heat; reduce heat, and simmer, stirring occasionally, 7 minutes. Add spinach leaves, and simmer 3 minutes. Remove from heat; cool slightly.
Process bell pepper mixture in a blender until very smooth. Stir in enough remaining chicken broth to reach sauce consistency, if necessary. Pour through a wire-mesh strainer into a bowl, discarding solids; stir in salt and pepper.